Silicon Controlled Rectifier Model Railroad Throttles

Published:2013/6/7 21:33:00 Author:muriel | Keyword: Silicon Controlled, Rectifier Model, Railroad Throttles | From:SeekIC

The following is a schematic drawing of a Silicon Controlled Rectifier type throttle for use on larger scale model railroads. Three versions of this throttle are shown on this page. They are not sophisticated designs but work well and are tough and reliable. It was designed for use on a The London Model Railroad Group's large O scale layout located at London, Ontario, Canada. The prime requirements for the club throttles were that they be rugged, reliable and produce as little heat as possible. Throttles of this type have been in service at the club since 1987 with excellent results. The throttle will deliver 5 amps continuously and up to 18 volts DC. See Notes. The Programmable Unijunction Transistor used to trigger the SCR is the key to this design as it ensures that the SCR fires on every cycle of the fullwave DC. This gives a very efficient operation with low voltage loss and very little heat generated. Please refer to your electronics books if you need more information on how the SCR and PUJT in the circuit function. The SCR throttle has momentum effect built in due to the memory provided by the 50uF capacitor at the PLUS input of the OPAMP. There is no decay but this can be added by placing a 1 to 2 Megohm resistor in parallel with the 50uF capacitor. Increasing the value of the memory capacitor will slow the action of the throttle controls. Due to its unregulated voltage output, the SCR throttle will have a prototypical feel as trains will slow when climbing grades and speed up when going down grades. The operator must work the throttle to keep their train from speeding or slowing too much in hilly terrain. SCR Throttle Setup Instructions The 50K Trimmer is adjusted so that maximum output voltage can be obtained but keep the SCR triggering on each cycle. To do this perform the following steps. 1. With no load connected to the throttle output and the AC input turned on. 2. Connect an analog voltmeter to the output of the throttle (0-30V). 3. Set the trimmer to its maximum resistance. 4. Press and hold the ACCELERATE push button until the output voltage stops increasing. 5. DECREASE the resistance of the trimmer until the output voltage peaks and then begins to fall again. When the voltmeter needle starts to bounce slightly the SCR is at its maximum trigger angle. STOP. 6. INCREASE the resistance of the trimmer slightly until the meter needle stops bouncing. At this setting the throttle will be able to deliver its maximum voltage. If the trimmer resistance is set too low the SCR will not trigger on every cycle of the full wave DC at full output voltage and could be damaged by high current spikes. Notes * NOTE 1 At full throttle and with no load this circuit will deliver more than 25 volts. Therefore care must be taken with auxiliary equipment and train lighting that may be connected to the track. (Remember that most voltmeters only indicate average or RMS voltage and not the peak voltage which can be 1.4 times greater that the average.) * NOTE 2 If your trains will run as fast as you need at lower track voltages than 18 volts then lower the input to 12 or 14 volts AC. The under load DC output voltage of this type of throttle is roughly equal to the RMS AC input voltage. * NOTE 3 Add your own reversing switches to the output and a suitable circuit breaker for added protection if desired. * NOTE 4 The use of good quality push buttons is a must as these are the most likely components of the throttle to fail due to the heavy thumbs of some people. Please feel free to use this design if you wish. It worked for our club it might be useful to you.



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